All mechanically moving objects emit sound at various frequencies, and acoustic screening chambers are known which can measure the sound intensities produced. Such measurements can be important as a research tool, as well as being used in product design, manufacturing, quality control, diagnostics and troubleshooting. Some examples of products for which sound emission is of current interest are computer disk drives, spindle motors and electronic transformers, all of which are often tested for the amount and frequencies of sound produced.
In general it is desirable to conduct acoustic screening in an environment having (1) a high signal to noise ratio, and (2) either an acoustic free measurement field or an acoustic diffuse measurement field. Acoustic free fields can be simulated in an anechoic or semi-anechoic chamber. Acoustic diffuse measurement fields can be simulated in a reverberation chamber. To achieve a high signal to noise ratio, the chamber is generally constructed with surfaces having a significantly large sound transmission loss. The signal to noise ratio is also related to the size of the chamber (height, width and length) and the sound power emission of the item under test. In the past, chambers simulating acoustic free measurement fields have varied in size from less than one cubic foot to more than twenty five thousand cubic feet, while chambers simulating acoustic diffuse measurement fields have typically been significantly greater than five hundred cubic feet and often times greater than 8,0000 cubic feet.
Previously known acoustic diffuse chambers thus suffer from inconvenient size. This problem has been addressed, but only at the cost of incurring additional problems. U.S. Pat. No. 4,051,917 to Grundmann, for example, addresses the excess size problem by including a sound-dampening liquid in the walls. While somewhat effective in reducing the overall size of the sound chamber, the use of liquid containing walls presents additional problems such as excessive weight and potential leakage. Another problem encountered in reducing chamber size is difficulty in providing accurate measurements. The amount of sound detected is always a function of the relative positions of the sound emitting object and the microphone, and this problem is exacerbated in small chambers. It is known to address this problem through the use of multiple microphones, but many microphones may be needed to provide adequate spatial averaging. Other problems may not be resolved due to acoustical standing waves within the chamber.
Thus, a need still exists to provide a small-sized, readily transportable, acoustic diffuse sound chambers, which nevertheless provides adequate acoustic characteristics for testing the amount and frequencies of sound generated by sound emitting devices.